1. Field of the Invention
The invention relates to electrophotographic copying machines. More particularly, the invention relates to a unique configuration for a belt type electrophotographic machine.
2. Prior Art
The electrophotographic copying process which is used primarily for reproducing copies of original documents is well known in the prior art. Although there are several variations to the process, the basic process requires a photoreceptor or photosensitive element. The photosensitive element is charged by a corona. The photosensitive element usually operates as an insulator in the dark and as a conductor in the light. After charging, the photosensitive element is next exposed to an imaging station. At the imaging station the photosensitive element is selectively discharged in accordance with the informational contents of an original document. The selectively discharge section of the photosensitive element contains a latent image of the original document. The latent image is developed by a developing material such as toner powder. The toned image is next transferred to a supporting media such as paper. The image is then fused into the paper to form a permanent copy of the original. The photosensitive element is cleaned and the above-enumerated process is repeated.
In order to selectively discharge the photosensitive element, the prior art uses either a flash illumination or a narrow footprint of light for scanning the original document. The flash illumination seems to be more efficient in that a complete footprint of the document is incident on the photosensitive element in a relatively short time interval. The short time interval is hereinafter called the scan cycle. As such, the throughput (that is, number of copies reproduced) of the electrophotographic copier can be significantly increased without changing the process speed of the other electrophotographic processing steps. On the other hand, when the footprint of light is used to scan the original document, only sections of the document are incident, sequentially, onto the photosensitive element. Also, the scan cycle includes a flyback time wherein no useful scanning is done. However, the flyback time is necessary so that the mechanism which generates the narrow footprint of light can be repositioned at its home position prior to the beginning of another scan cycle. The net result is that the time required to scan an original document is increased and the throughput on such an electrophotographic copier is reduced.
Prior art electrophotographic copiers which reproduce copies by using the above-enumerated process may be divided into two classes; the so-called belt type electrophotographic copiers and the drum type electrophotographic copier. Each type of the prior art electrophotographic copier will be discussed hereinafter with particular emphasis on the shortcomings associated with each type.
U.S. Pat. No. 4,089,516 is an example of the drum type prior art electrophotographic copier. The copier includes a cylindrical drum with a length of photosensitive material wrapped about its surface. The drum is journaled for rotation about a shaft which is mounted to the frame of the copier. A plurality of processing stations (including charging, imaging, developing, transferring and cleaning) are positioned around the periphery of the cylindrical drum. In order to copy an original document, the document is placed on the document platen. A narrow footprint of light is generated by the illumination system. The footprint of light scans the document and the light rays reflecting from the document are focused through the optical path of the system to form a latent image onto the photosensitive material. The latent image is developed and transferred to a support media. The image on the support media is fused by a heat source to form a permanent copy.
This type of prior art electrophotographic copier has several meritorious characteristics. For example, compactness of the machine. Generally less space is required to package the copier since the processing stations are arranged in a circular configuration. Also, cleaning and stripping the transfer media from the photosensitive element is done more efficiently since the photosensitive element has a continuous curvature. Perhaps the nonmeritorious characteristic of the system is that flash illumination cannot be used to discharge the photosensitive material. The reason is that the photosensitive material is curved and only a portion of the photosensitive material can be discharged at any instant of time. As a result, the throughput of this type of electrophotographic copier is usually relatively low.
U.S. Pat. No. 3,661,452 exemplifies a belt type prior art electrophotographic copier. In this type of copier, an endless photosensitive belt is supported for travel, in an endless generally triangular path, by a plurality of rollers. At least one of the rollers is biased, by mechanical means such as a spring, to create tension in the belt. a plurality of conventional processing stations are positioned in proximity within the orbit of the photosensitive belt. The processing station includes a flash illumination system for exposing the photosensitive belt on a first flat run. The exposed photosensitive belt is developed by a development station which is positioned relative to a second flat run of the belt. At least one of the rollers has a sufficiently wide diameter to curve the belt at a point between the flash illumination system and the development station. Image transfer from the belt to a supporting media, such as paper, occurs at the curved section of the belt.
Although the above-described belt type electrophotographic copier is an improvement to the prior art in that it incorporates one feature (namely, image transfer at a curved surface) from the prior art drum type copier, it has a few drawbacks. For example, the optical path includes a plurality of mirrors which reflect light emitting from the document glass onto the photosensitive belt. As the light rays are reflected from mirror to mirror, some of the light intensity is lost. Unless the light rays which strike the photosensitive belt are of a certain intensity, the belt is not properly discharged and the copy quality tends to be poor. The mirrors which are used to reflect the light rays often require optical alignment. Optical alignment is often difficult to achieve and expensive. A large number of the belt type copiers use toner for developing a latent image on the photosensitive belt. The toner tends to contaminate the mirror and adversely affects their light reflecting characteristics. Finally, it is often difficult to replace photosensitive belt in the above described type of copier. The difficulty stems from the fact that the mechanical biasing means which tension the belt has to be adjusted before the belt can be removed.